Silicone adhesive compositions can be pressure sensitive or permanent bonding types of adhesives. Permanent bonding implies that the adhesive will actually cement two surfaces together, i.e., it behaves like a glue. Pressure sensitive, on the other hand, means that the adhesive can be stripped from a surface and re-adhered to the surface, i.e., it has the nature of the adhesive present on Scotch® Brand tapes. The adhesive of the invention are pressure sensitive adhesives but can be formed into permanent bonding adhesives when combined with a suitable crosslinking agent such as a hydrogen bearing silicone polymer, and a catalyst.
Typically, the components of a silicone pressure sensitive adhesive consist of (i) a silicone resin containing monofunctional (M) units R3SiO1/2 and tetrafunctional (Q) units SiO4, i.e., an MQ silicone resin, wherein R is a hydrocarbon group such as methyl; and (ii) a high molecular weight hydroxyl endblocked polydiorganosiloxane fluid with a viscosity of 5,000 to 1,000,000 mm2/s at 25° C., or a high molecular weight hydroxyl endblocked polydiorganosiloxane gum where viscosity is expressed in terms of plasticity.
Silicone pressure sensitive adhesives can be prepared by simply mixing components (i) and (ii). Generally, this takes place in the presence of a mutual solvent such an organic, aromatic or a hydrocarbon solvent, i.e., ethyl acetate, heptane, xylene, or toluene. However, the solvent can be omitted. As soon as components (i) and (ii) mixed, the composition is ready for use as a pressure sensitive adhesive without further treatment. It can simply be applied to the surfaces to be adhered by any suitable means, and then the surfaces are brought together. Typically, if the composition contains a solvent, the solvent is allowed to evaporate before adhering the two surfaces. The coating can be cured for a short time by heating it briefly, although curing is not generally required. Likewise, a catalyst can be added to assist in the curing, although a catalyst is not generally required.
Silicone pressure sensitive adhesives are described in U.S. Pat. No. 2,736,721 (Feb. 28, 1956); U.S. Pat. No. 2,814,601 (Nov. 26, 1957); U.S. Pat. No. 2,857,356 (Oct. 21, 1958); U.S. Pat. No. 4,584,355 (Apr. 22, 1986); U.S. Pat. No. 4,585,836 (Apr. 29, 1986); U.S. Pat. No. 4,591,622 (May 27, 1986); U.S. Pat. No. 4,655,767 (Apr. 7, 1987): and U.S. Pat. No. 5,482,988 (Jan. 9, 1996). In addition, other types of adhesives having a more suitable surface pressure sensitive adhesion property can be used, such as the so-called Soft Skin Adhesive, i.e., the siloxane gel compositions described in detail in U.S. Pat. No. 5,145,933 (Sep. 8, 1992), which are prepared from (A) alkenyl-containing polydiorganosiloxanes, (B) hydrosilicon compounds having at least three SiH groups, (C) SiH end-blocked polydiorganosiloxanes, and a (D) catalyst. None of these patents, however, either describe or suggest the silicone pressure sensitive adhesive compositions according to the invention that are capable of adhering to wet surfaces.